Binomial Distribution, balls and questions

I have 2, two part questions.

1. A box contains 3 yellow and 5 black balls. A ball is drawn and then replaced. This process is continued for a total of 5 draws. what is the probability of drawing:
a. exactly 4 black balls
b. at most 4 black balls

2. A test contains 20 multiple choice questions each with 3 choices. If a student guesses every answer, what is the probability he will get:
a. all the answers wrong
b. exactly 5 correct answers.